Abstract

A recently proposed model for metastable production of blueschists during serpcntimzation has broad implications for processes taking place at tectonic continental borders. This paper accepts current ideas on continental underthrustmg by the sea-floor spreading mechanism and examines the following problems associated with the Franciscan Formation: (1) presence of amphibohtes; (2) detrital serpentine and glaucophane; (3) origin of alpine ultramafic rocks; and (4) time relationships of sedimentation and metamorphism. The metastable formation of blueschists at relatively shallow depths is more compatible with these observations than the deep burial required by conventional facies treatment of metamorphism.

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